The present invention relates to electronic solid state, i.e., static, trip units for industrial circuit breakers, and particularly to long-time and short-time overcurrent signal processing circuitry therefor.
Static trip units are being implemented in increasing numbers in industrial circuit breakers in place of the traditional thermal and magnetic trip units to initiate automatic circuit interruption by the circuit breaker in response to an overcurrent condition. The increasing popularity of static trip circuit breakers is largely due to their inherent versatility. That is, a static trip unit is readily susceptible to convenient trip setting adjustment such that a highly repeatable trip-time curve can be rather precisely tailored to a particular load so as to provide full over-current protection ranging from light overload to heavy short circuit proportions without nuisance tripping the circuit breaker in response to expected overcurrent conditions of a temporary nature. Having these capabilities, a static trip circuit breaker is quite amenable to selective trip coordination with the overcurrent response characteristics of circuit protective devices upstream and downstream thereto.
Typically, static trip units include a so-called "long-time" trip mode network and a so-called "short-time" trip mode network operating in coordination to jointly process an overcurrent signal indicative of an overcurrent condition on the distribution circuit to be protected. The long-time trip mode network is adjusted to establish an appropriate pickup level against which the signal indicative of the highest current flowing in any one of the distribution current conductors is measured to determine if an overcurrent condition exists. If the signal exceeds the selectively established long-time pickup level, the long-time trip mode network begins timing out a long-time delay before initiating circuit breaker tripping on an inverse time basis, preferably in accordance with the equation I.sup.2 t=constant. Thus, the lower the overcurrent magnitude, the longer the delay to breaker tripping, and vice versa.
While the long-time trip mode network is devoted to protecting the distribution circuit against overcurrent conditions of overload proportions, the short-time trip mode network protects against higher levels of overcurrent from heavy overloads and upward. Thus, the short-time trip mode network is adjusted to establish an appropriate pickup level in excess of the long-time pickup level. If the overcurrent signal exceeds this elevated pickup level, the short-time trip mode network begins timing out a preselected fixed short-time delay before tripping the circuit breaker.
A typical static trip unit would also include a so-called "instantaneous" trip mode network having a pick-up level in excess of the short-time pickup level, which, if exceeded by the overcurrent signal, manifests a heavy short circuit condition on the distribution circuit. The instantaneous trip mode network, in response, initiates tripping of the circuit breaker essentially instantaneously, i.e., without intentional delay.
It is an object of the present invention to provide a circuit breaker static trip unit having improved overcurrent signal processing circuitry for performing the long-time and short-time trip functions.
Another object is to provide overcurrent signal processing circuitry of the above character having long-time and short-time trip mode networks operating in a uniquely coordinated manner to jointly process the overcurrent signals.
A further object is to provide overcurrent signal processing circuitry of the above character providing an adjustably selected circuit breaker time-current response characteristic capable of improved trip selective coordination with upstream and downstream circuit protective devices.
An additional object is to provide overcurrent signal processing circuitry of the above character operating to indicate when the long-time trip mode network is timing out a long-time delay.
Yet another object is to provide overcurrent signal processing circuitry of the above character accommodating zone selective interlocking with other like circuit breakers upstream and downstream thereto.
A still further object is to provide overcurrent signal processing circuitry of the above character which is efficient in construction and versatile in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.